While this can be caused by several factors, the most common cause is improper thinner usage. A mottled look or “blushing” effect is often caused when the thinner in the paint dries too quickly. When this happens, as the thinner evaporates, it re-condenses and falls, or literally “rains”, back on to the finished face rewetting the paint. To help eliminate this, use a “slower” thinner or mix some of the slower thinner in to the paint with the regular thinner. This will retard the paint from drying too quickly and help eliminate the blushing effect.